The principles of sound workholder design

Modern Machine Shop, May, 1995

Though there are many variations of workholders, each must adhere to a few basic design principles to guarantee the clamp's proper operation and overall safety. Locating and clamping are two general areas of workholder design that should be completely understood to help ensure a successful design.

Three Locating Functions

To properly accomplish its purpose, a workholder must ensure the repeatability of the workholding function. In the case of jigs and fixtures, this means properly positioning, or locating, a workpiece, time after time, within the stated limits. Locating is therefore a critical function of any workholding device, and the design of locating elements must be made as efficient and effective as possible.

The locators must actually perform three fundamental functions: to hold, support and locate the workpiece. Because accomplishing these functions ensures the repeatability of the manufacturing process, each of the functions should be fully understood.

* Holding the workpiece:

While holding the workpiece is often thought of as a function of the clamping device, it is actually the responsibility of the locators. In addition to properly positioning the workpiece, one of the main functions of the locators is to prevent any workpiece movement caused by the forces of the cutting tool acting on the workpiece.

The real purpose of the clamps in a workholder is to hold the workpiece against the locators. When properly applied, the locators provide a positive stop to resist workpiece movement.

* Supporting the workpiece:

Generally speaking, supporting a workpiece, when applied to work-holding, simply refers to the positioning of locators under the workpiece, as referenced on the Z axis. Like the workpiece holding function, the support function is also the responsibility of the locators.

The different axes of a workpiece are usually identified by a series of letters. The most common identifications are X, Y and Z. The X axis of the workpiece generally describes the longitudinal horizontal axis. The Y axis identifies the traverse horizontal axis. And the Z axis indicates the vertical axis of the workpiece.

* Locating the workpiece:

Locating, as applied to work-holding, is simply the process of properly positioning the work-piece in the workholder in relation to its horizontal X and Y axes.

From the functions described, you can see that the locators used in any jig or fixture serve several different, yet related, purposes.

Designing A Locating System

To properly design a locating method for a workpiece, you first need to understand exactly what the locating process involves. This means understanding the degrees of freedom of a workpiece. By learning how this concept is applied, you can begin to understand the methods you can use to make your workpiece location as efficient as possible.

Any workpiece, free in space, is capable of moving in an infinite number of directions. However, to make workpiece locating simpler, the overall movements any workpiece can make have been reduced to only twelve degrees of freedom. As shown in Figure 1, these consist of six radial degrees of freedom and six axial degrees of freedom. The radial degrees of freedom permit rotational movement around each axis, while the axial degrees of freedom permit movement along each axis.

Since a workpiece typically has three axes, it can also be said that each axis has four degrees of freedom, as shown. The simplest way to restrict this movement is by the six-point, or 3-2-1, locational method. By using these six points to locate a workpiece, all but three degrees of freedom can be restricted. In addition, the six-point location method is the most efficient and accurate method of locating any workpiece by three of its sides.

As shown in Figure 2, the primary workpiece surface is located on three points, which establish a plane. Once these points are located, five degrees of freedom are restricted. The workpiece cannot move down, nor can it rotate in either direction about either the X or Y axes.

The secondary locating surface is located on two points. This restricts three more degrees of freedom, for a total of eight. These two locators restrict axial movement in one direction on the Y axis and the radial movement around the Z axis.

The final locating point, on the tertiary surface, restricts one additional degree of freedom - the axial movement in one direction along the X axis. Added together, the six locators restrict a total of nine degrees of freedom. The remaining three degrees of freedom are contained by the clamping device.

The six-point locational method is best suited for rectangular workpieces that are located by their outer edges. However, if the workpiece to be located has one or two holes that can be used for location, these may be even more efficient and effective.

While any of several methods can be used to locate a workpiece, the overall goal is to employ the most effective and efficient means. The workpiece itself will normally determine the locating methods in designing a jig or fixture. But, for those situations where several options are available, it is a good idea to know the order of preference for various locating features. These are listed below in descending order of efficiency, effectiveness and locating accuracy: